Generally, when a packet is transmitted in a mobile communication system, a receiver should inform a transmitter of a success or failure in packet reception. If the packet reception is successful, the receiver transmits ACK to enable the transmitter to transmit a new packet. If the packet reception is not successful, the receiver transmits NACK to enable the transmitter to retransmit the corresponding packet. This operation may be called ARQ (automatic request).
This ARQ operation may be combined with a channel coding scheme. In particular, the above-mentioned ARQ has been proposed as HARQ (hybrid ARQ) for raising system efficiency by lowering an error rate in a manner of combining a retransmitted packet with a previously received packet. In order to raise system throughput, the HARQ is requested to receive a ACK/NACK response from the receiver quicker than that of the ARQ operation. Hence, ACK/NACK in HARQ is transmitted by physical channel signaling.
Implementation of HARQ may be mainly into two types. A first type of HARQ implementation is CC (chase combining). In particular, when a retransmission is performed, a retransmitted packet is transmitted in a manner of having the same code bits using the same modulation scheme of a previously transmitted packet and the same coding rate of the previously transmitted packet. A second type of HARQ is IR (incremental redundancy). In particular, when a retransmission is performed, a transmission of code bits different from those of a previously transmitted packet is granted using a modulation scheme and coding rate different from those of the previously transmitted packet. In this case, a receiver may be able to raise system throughput by coding diversity.
In a multicarrier cellular mobile communication system, user equipments belonging to one or plural cells perform uplink (UL) data packet transmissions to a base station. Since a plurality of user equipments are able to transmit UL data packets in a single subframe, a base station needs to transmit ACK/NACK signals to a plurality of the user equipments in a single subframe as well. In particular, in 3GPP LTE system, a base station transmits ACK/NACK signals to a plurality of user equipments on a physical HARQ (hybrid ARQ) indicator channel (hereinafter abbreviated PHICH), i.e., a channel for carrying downlink (DL) ACK/NACK information for UL HARQ.
When a plurality of ACK/NACK signals transmitted by a base station to user equipments in a single subframe are multiplexed by CDMA in a partial time-frequency domain of a DL transmission band of a multicarrier system, they are discriminated from ACK/NACK signals for other user equipments by an orthogonal code or a pseudo orthogonal code multiplied in a time-frequency domain. Moreover, when QPSK transmission is performed, they may be divided into two different orthogonal phase components. In particular, in 3GPP LTE system, a plurality of ACK/NACK signals are transmitted on a plurality of PHICHs by being multiplexed by CDMA. In doing so, a unit of a multiplexed transmission by CDMA is called a PHICH group.
Meanwhile, in case that a specific user equipment attempts an initial access to a prescribed cell, it is necessary for the specific user equipment to acquire system information. Such a basic information in the system information as a system bandwidth and the like may be received on a physical broadcast channel (hereinafter abbreviated PBCH). Yet, in order to acquire detailed system information in the system information of a corresponding cell, it may be necessary for a user equipment to receive a physical downlink shared channel (hereinafter abbreviated PDSCH) for carrying general DL data.
In doing so, since scheduling information of PDSCH is carried on PDCCH of each subframe, a user equipment in the course of an initial access receives PBCH and then receives PDCCH of a specific subframe to recognize scheduling information on PDSCH for carrying detailed system information through the specific subframe. In particular, in order to receive the PDCCH having the scheduling information on the PDDSCH for carrying the detailed system information, the user equipment should be aware of a transmission position of the corresponding PDCCH.
Since PDCCH is generally mapped to a resource element (hereinafter abbreviated RE) except resource elements for carrying PHICH and other control signals, the user equipment should be aware how the PHICH and other control signals are mapped to a resource region, in order to receive the PDCCH.